Introduce a flag for tracing retaining path in GC.

src/heap/mark-compact.h

Index: src/heap/mark-compact.h
diff --git a/src/heap/mark-compact.h b/src/heap/mark-compact.h
index 50c23dd810d7d17552f5dcdd9392711a640b5065..4e3352baba0b2b027a6d8fb05d3cfc5e5f395d1c 100644
--- a/src/heap/mark-compact.h
+++ b/src/heap/mark-compact.h
@@ -135,6 +135,7 @@ class Marking {
   Heap* heap_;
 };
 
+
 // ----------------------------------------------------------------------------
 // Marking deque for tracing live objects.
 class MarkingDeque {
@@ -167,6 +168,9 @@ class MarkingDeque {
   // otherwise mark the object as overflowed and wait for a rescan of the
   // heap.
   INLINE(void PushBlack(HeapObject* object)) {
+#ifdef TRACE_RETAINING_PATH
+    AddRetainedObject(object);
+#endif
     DCHECK(object->IsHeapObject());
     // TODO(jochen): Remove again before we branch for 4.2.
     CHECK(object->IsHeapObject() && object->map()->IsMap());
@@ -181,6 +185,9 @@ class MarkingDeque {
   }
 
   INLINE(void PushGrey(HeapObject* object)) {
+#ifdef TRACE_RETAINING_PATH
+    AddRetainedObject(object);
+#endif
     DCHECK(object->IsHeapObject());
     // TODO(jochen): Remove again before we branch for 4.2.
     CHECK(object->IsHeapObject() && object->map()->IsMap());
@@ -217,6 +224,10 @@ class MarkingDeque {
   void set_top(int top) { top_ = top; }
 
  private:
+#ifdef TRACE_RETAINING_PATH
+  static void AddRetainedObject(HeapObject* obj);
+#endif
+
   HeapObject** array_;
   // array_[(top - 1) & mask_] is the top element in the deque.  The Deque is
   // empty when top_ == bottom_.  It is full when top_ + 1 == bottom